From Longman Dictionary of Contemporary Englishslick something ↔ down/back phrasal verbSHINYto make hair smooth and shiny by putting oil, water etc on it His hair had been combed back and slicked down with something to make it neat. → slick→ See Verb table
Examples from the Corpus
slick down/back• Michael had time to put Conker out before running in to wash his hands and slick his hair back wetly. 